This invention relates to a method of damping vibrations transmitted from an engine of a small watercraft to its hull.
In the case of small watercraft such as motorboats and fishing boats, it was of secondary importance to damp engine vibration because the travel time and distance of such boats are usually far shorter than large ships. Thus, many older small ships have their engine mounted directly on an iron bed provided on the bottom of the hull. It is a recent trend, however, to mount the engine on an iron bed not directly, but through a rubber mount.
Such a rubber mount is a mounting member for supporting mounting legs provided parallelly on the engine. It includes a fixing member fixed to the bed, a rubber member inserted in the fixing member, and a mounting means comprising a bolt extending through the rubber member and a nut for fixing the bolt to the mounting leg of the engine by tightening it. The rubber member serves to absorb any vibration of the leg in vertical and horizontal directions.
On the other hand, on a propeller shaft for transmitting the engine torque to a propeller, a force acts counteracting the force of the propeller for propelling the ship forward. This reaction force acts in such a way as to move the engine relative to the hulls, so that the engine fixing means tends to loosen. Also, this force acts repeatedly on the engine as a thrust force. One way to damp this force is shown in FIGS. 6A, 6B.
The arrangement shown in FIG. 6A comprises a main engine unit 1 including an engine 2, a transmission 3 including a thrust bearing, a universal joint 3', another thrust bearing 3", a propeller shaft 4 and a propeller 5.
In the arrangement shown in FIG. 6B, the engine torque is transmitted through the transmission 3 to the propeller shaft 4, while the reaction force is dispersed by the thrust bearing 3".
With the arrangement that utilizes a rubber mount, it is possible to effectively damp vibrations transmitted from the engine to the hull while the ship speed is low. But as the ship speed increases to high speeds, it becomes difficult to damp engine vibrations. Thus, engine vibrations transmitted to the hull tend to grow to such an extent as to give unpleasant or unbearable feelings to people on board the ship.
This is because the rubber member loses its vibration damping functions at higher ship speeds. More specifically, as the ship speed increases to a certain point, the rubber member in the fixing means of the engine supporting rubber mount is pushed to one side due to the reaction force of the ship propelling force to such an extent that the fixing bolt abuts the fixing means. Once this happens, the rubber member cannot damp the horizontal component of the engine vibration any more.
One possible solution to this problem is to mount the thrust bearing shown in FIGS. 6A, 6B at an intermediate portion of the propeller shaft. But such a thrust bearing cannot reduce the displacement of the engine so dramatically. Moreover, this arrangement tends to be complicated in mechanism and thus costly.